The present invention relates generally to magnetic resonance imaging (MRI) apparatus, and more particularly to, a system and method to provide improved cooling to an MRI apparatus and thereby enable higher power applications for faster imaging with improved image quality and allow longer scan times.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
During patient scans, the gradient coils that produce the magnetic field gradients dissipate large amounts of heat, typically on the order of tens of kilowatts. The majority of this heat is generated by resistive heating of the copper electrical conductors that form the x, y, and z-axis gradient coils when these coils are energized. The amount of heat generated is in direct proportion to the electrical power supplied to the gradient coils. The large power dissipations not only result in increased temperature to the gradient coil, the heat produced is distributed within the gradient coil assembly, or resonance modules, and influences the temperature in two other critical regions. These two regions are located at the boundaries of the gradient assembly and include the patient bore surface and the warm bore surface adjacent to the cryostat that houses the magnets. Each of these three regions has specific maximum temperature limitations. In the resonance module, there are material temperature limitations, such as the glass transition temperature. That is, although the copper and fiber-reinforced backing of the coils can tolerate temperatures in excess of 120° C., the epoxy to bond the layers typically has a much lower maximum working temperature of approximately 70–100° C. On the patient bore surface, regulatory limits mandate a peak temperature on the patient bore surface of 41° C. The warm bore surface also has a maximum temperature that is limited to approximately 40° C. to prevent excessive heat transference through the warm bore surface and into the cryostat. Further, temperature changes of more than 20° C. can cause field homogeneity variations due to a temperature dependence of the field shim material that exhibits a magnetic property variation with temperature.
Typically, the heat produced by the gradient coils in the resonance modules is removed from the gradient assembly by liquid filled cooling tubes embedded in the resonance modules at a given distance from the heat conductors. A liquid coolant, such as water, ethylene, or a propylene glycol mixture, enters the resonance module at a fixed temperature and flow rate, absorbs heat from the gradient coils as it is pumped through the cooling tubes, and transports the heat to a remote heat exchanger/water chiller. Heat is then ejected to the atmosphere by way of the heat exchanger/chiller. For each degree reduction of the coolant temperature as it enters the resonance module, the peak temperatures at each of the three critical regions (resonance module interior, patient bore surface, and warm bore surface) are also lowered.
However, in current systems, the minimum temperature of the coolant supplied to the resonance modules is limited by the dew point temperature of the ambient air. That is, since it is necessary to prevent the water vapor in the air from condensing in the resonance modules in general, and on the gradient coils in particular, the temperature of the coolant must remain above the dew point temperature of the ambient air. The high voltages and currents that are applied to the gradient coils dictates an atmosphere that must be free of such condensation. Current environmental specifications for MR rooms require 75% relative humidity at 21° C., which requires a dew point temperature of 16° C. Therefore, the minimum coolant temperature must be above 16° C. under these conditions.
The maximum power which can be supplied to a resonance module is therefore limited by the external dew point temperature. To increase the power which can be received by the resonance module, it is necessary to lower the minimum coolant temperature. However, as indicated previously, environmental specifications limit the minimum coolant temperature to above 16° C. for an MR room with 75% relative humidity at 21° C. As a result, these current systems are unable to accommodate higher power patient scan sequences often required by resonance modules.
In these known systems, the lowest permissible coolant temperature is dictated by atmospheric conditions or the ambient dew point temperature. With these systems, the coolant temperature is set above the worst case dew point temperature based upon the given temperature and relative humidity specifications in the room housing the MR system.
Further, these systems must be kept from overheating. In case of increased temperatures of the resonance module or the patient surface, imaging scans must be interrupted or limited to low power sequences, which in turn reduces the efficiency and efficacy of the MR system. Time is then lost because imaging sessions cannot begin anew until the resonance module or patient surface cools sufficiently.
It would therefore be desirable to design a method and system to dissipate more heat during imaging scans independent of the aforementioned minimum coolant temperature limitation dictated by the dew point temperature of the ambient air.